A kinetic study of the exsolution of pentlandite (Ni,Fe)₉S₈from the monosulfide solid solution (Fe,Ni)S

Abstract

The kinetics of the exsolution of pentlandite from the monosulfide solid solution (mss) have been investigated using a series of anneal/quench and in situ cooling neutron diffraction experiments. Five mss compositions were examined by anneal/quench techniques covering the composition range Fe_0.9Ni_0.1S to Fe_0.65Ni_0.35S and using annealing temperatures between 423 and 773 K for periods from I h to 5 months. In situ cooling experiments were performed on four mss compositions in the range Fe_0.9Ni_0.1S to Fe_0.7Ni_0.3S. The samples of these solid solutions were heated to 973 K, and then cooled to 373 K in steps of 50 K over a 24 h period. The extent of exsolution was monitored by Rietveld phase analysis using powder neutron diffraction data. The anneal/quench experiments established that initial exsolution of pentlandite from mss above 573 K is very rapid and is effectively complete within 1 h of annealing. However, the mss/pyrrhotite compositions remained Ni rich (17 at% Ni) after 5 months annealing, indicating that compositional readjustment at low-temperatures occurs over long periods. Below 573 K, exsolution is less rapid with rate constants in the range 6 × 1O^-6 to 1 × 10^-5/s and the activation energy for exsolution of pentlandite from mss Fe_0.5Ni_0.2S between 473 and 423 K is 5 kJ/mol.

The in situ cooling experiments showed that the temperature at which exsolution commences upon cooling decreases from 873 K for Fe_0.7Ni_0.3S to 823 K for Fe_0.9Ni_0.1S and that exsolution effectively ceased on the time scale of the experiments at temperatures between 598 and 548 K. The kinetic data were analyzed using the Avrami model where y = 1 - exp(-kⁿtⁿ) and the initial rates of exsolution were found to increase with Ni content from 2 × 1O^-6Zs for Fe_0.9Ni_0.3S to 4 × 10^-5/s for Fe_0.7Ni_0.3S. Both high Ni content and high M: S ratio served to facilitate nucleation rate, indicating that nucleation occurs at S vacancies within mss crystals rather than at grain boundaries. Values of the Avrami geometric constant n vary during exsolution upon cooling indicating three possible changes in the growth mechanism during the reaction. The roles of impurities and S fugacity on reaction rates are discussed. The rate constants for exsolution of pentlandite from mss/pyrrhotite in nature are estimated to be 4 or 5 orders of magnitude slower than those reported here, still very rapid on a geological time scale. High metal mobility persists in this system at low temperatures, even at room temperature, and the textures and compositions observed in nature are a consequence of very low-temperature (<100 ⁰C) equilibration of assemblages over geological time scales.